ABSTRACT
The synthesis of immunoglobulins (Igs) is unique by the fact that heavy and light chains are synthesized independently and assembled postsynthetically in a single protein. Polymeric Igs have a unique protein, J chain, that interacts with a receptor in mucosal cells and is then incorporated in the secreted molecules. Immunoglobulins have different synthetic and catabolic rates, the sum of which is reflected in the Ig half-life, longer for IgG1 (21 days) and shorter for IgE (2.4 days). Most biological properties of immunoglobulins depend on the ability of the Fc region to engage enzymatic systems or react with Fc receptors. IgG and IgM are most efficient in the activation of the complement system. IgG is the only immunoglobulin that binds to the FcRn and crosses the human placenta. IgG also binds to the Fc receptors in phagocytic cells, causing their activation and the release of inflammatory mediators. IgG is also recognized by cells targeting cells for natural killer (NK) cell–mediated antibody-dependent cellular cytotoxicity. Finally, IgE binds to high-affinity receptors in basophils and mast cells, and if cross-linked by the antigen that has induced the IgE antibody, will cause the release of histamine and other mediators that cause allergic reactions.
